Two-component epoxy resin compositions used as coatings can consist of a resin component which comprises an epoxy resin, and of a hardener component which comprises compounds that are reactive with epoxy groups, for example, primarily polyamines. The two components are mixed for use, and they cure at ambient temperature. Systems can have several properties that are undesirable in practice.
Epoxy resins, for example, those based on bisphenols, are viscous fluids or solids. For use in two-component epoxy resin compositions for coatings, they can be diluted to achieve a good miscibility of the two components, and a good processability at ambient temperatures. For the dilution, low-viscosity epoxies, so-called epoxy reactive diluents, can be used. However, they can be expensive, they can have a strong irritating effect, and they can lower the strength of the cured system in an undesirable manner. The epoxy resins can also be diluted with organic solvents. However, the latter can be volatile organic compounds (VOC; Volatile Organic Compound) which, during and after the processing of the epoxy resin composition, enter into the atmosphere due to evaporation, and can represent a hazard for humans and for the environment, because they can be highly flammable, they can cause odor emissions, and they can be capable of causing nausea, injuries to health, and environmental damage.
The polyamines that can be used as hardeners can present, due to their content of primary amino groups, the undesirable characteristic that they are capable of forming stable carbonate and carbamate salts, with gaseous carbon dioxide (CO2) from the air in combination with moisture. Hardeners that are based on such polyamines can, for example, not be stored open in the air, because crusts form on the container. Such hardeners, even when they are used in a coating—after mixing in the epoxy resin and during the curing—can be capable of taking up CO2, which can result in undesired effects, such as a tacky surface with tarnishing or spotting, and incomplete curing. These effects are referred to as “blushing” by the person skilled in the art. To suppress crust formation and blushing, and at the same time to achieve dilution, epoxy resin compositions can be mixed with considerable quantities of benzyl alcohol, which can result in undesirable effects. For example, although benzyl alcohol is relatively free of odors and presents low volatility, it is nonetheless a VOC, and consequently benzyl alcohol-containing composition cannot be declared to be VOC-free. Epoxy resin compositions having a high content of benzyl alcohol, moreover, tend to form bubbles, for example, on porous substrates, they have a reduced resistance to abrasion, and they can perform poorly in emission tests in interior spaces. Alkyl phenols, such as nonyl phenol, are also used to suppress blushing, but they can be problematic for reasons pertaining to toxicity.
Secondary amino groups do not undergo a spontaneous reaction with CO2, or their carbonates and carbamates are not stable. Polyamines having primarily secondary amino groups therefore can lead to little blushing in epoxy resin compositions. However, polyamines comprising only secondary amino groups are rarely used, because they can be expensive to produce, and lead to long curing times. Polyamines with adducted diepoxies, as well as so-called polyamidoamines, can be used. Both comprise besides primarily secondary amino groups also primary amino groups, and they can exhibit little blushing. However, their viscosity can be so high that they have to be diluted, which again can entail the mentioned undesirable effects.